Osmotherapy with mannitol remains the first treatment currently recommended to treat intracranial hypertension in severe head injury.However, this treatment in not always efficient and is associated with side-effects and a transitory action. Hypertonic sodium-lactate, by its osmotic and energetic properties could be superior to mannitol to decrease ICP. Study objectives : to compare the effect of mannitol and sodium-lactate on ICP at the fourth hour after the start of infusion during IHE in severe traumatic brain injury (TBI) (main endpoint) ; to compare the percentage of successfully treated episodes between both treatments and to compare the neurological status after one year of evolution (glasgow outcome score)(secondary endpoints)